The reversible thiol-disulfide reaction is a central theme in biology: it is used as a general strategy to accept and donate electrons and thus, participates in numerous redox reactions within the cell. The thioredoxin (Trx) and glutathione systems are the major thiol-dependent redox systems present in cells. These systems have overlapping yet distinct properties and cellular targets, but function in a similar manner: they transfer, via the reversible oxidoreduction of thiols, the reducing equivalents of NADPH to numerous substrates and substrate reductases. Through this mechanism both systems provide electrons to essential enzymes, maintain the cellular redox homeostasis and provide major defenses against oxidative stress. They also exert redox control of regulatory proteins, such as kinases, phosphatases and transcription factors involved in signal transduction and gene transcription. A molecular link between both systems has recently been described: thioredoxin glutathione reductase (TGR), a selenoprotein oxidoreductase that possesses thioredoxin reductase, glutathione reductase and glutaredoxin activities, achieving this broad substrate specificity by a fusion of glutaredoxin (Grx) and Trx reductase (TR) domains. The purpose of this study is to functionally characterize the TGR from the cestode parasite Echinococcus granulosus, the causative agent of hydatid disease. The project will be guided by the following specific aims (questions): i) how mitochondrial and cytosolic variants of this enzyme are generated from a single gene, ii) is translocation of the mitochondrial variant of TGR a redox-dependent event, and iii) what is the role of the selenocysteine-containing carboxy-terminal redox center and the glutaredoxin domain of TGR in electron transfer towards oxidized glutathione and glutaredoxin targets. A combination of molecular, biochemical and cell biology approaches will be used to address these questions. It is expected that the results will have a broader impact, contributing to the understanding of cellular redox reactions involving the amino acids cysteine and selenocysteine; in particular their roles as electron transfer devices and as molecular switches that control protein function and localization. This research will be done primarily in Uruguay at a Uruguayan University (Universidad de la Republica) in collaboration with Dr. Gustavo Salinas as an extension of NIH grant R01 GM065204.